1. Field of the Invention
The present invention relates to a turbocharged engine comprising an EGR unit having an EGR passage linking an intake passage and an exhaust passage and an EGR valve provided in the EGR passage.
2. Description of the Related Art
An EGR unit (Exhaust Gas Recirculation unit) is generally known as a device for reducing the amount of NOx (nitrogen oxides) in exhaust gas. The EGR unit comprises an EGR passage linking an intake passage and an exhaust passage and an EGR valve provided in the EGR passage.
The EGR unit opens the EGR valve, causes part of the exhaust gas present inside the exhaust passage to flow into the intake passage via the EGR passage, mixes this exhaust gas with the intake air, and guides the mixture into the combustion chamber, thereby reducing the concentration of oxygen in the intake air, lowering the combustion temperature, and decreasing the amount of discharged NOx.
Such an EGR unit is used in the turbocharged engine as well (see, for example, Japanese Patent Application Laid-open No. 2001-165000).
Usually in such an EGR unit, when a vehicle is decelerating, because the accelerator is off and fuel injection is not conducted, the EGR valve is fully closed.
However, in a turbocharged engine, when a vehicle is decelerating, a burst pressure sometimes remains inside the intake passage and the intake pressure sometimes crosses the surge line of the turbocharger compressor. If the surge line is crossed, surging is induced in the compressor, creating a risk of damage caused by vibrations or heat.